The regulation of the organism has traditionally been ascribed to two distinct systems-the nervous and the endocrine. Though coordination between the two systems has been acknowledged, researchers and authors have tended to deal with them as comprising separate categories of cells involved in different activities. With this approach, a given regulatory mechanism would be evaluated as to whether it should be accounted for by nervous or endocrine functions. The past 15 years, however, have witnessed numerous important discoveries and conceptual developments concerning the morphological,...